If it's your startup volume than you will have to put in your Tiger (or system resore) Disc, restart from the Disc (hold down the option key) and select the disc when it comes up) On the installer go to utilities>disc utility (somewhere in the menu bars) select the drive you want to erase, and click on it, then click on the erase tab, enter in your options and then click erase.
If it's an external drive, than simply go to Applications>Utilities>Diask Utility, click on the drive in the left-hand list, go to the erase tab, enter in the detainls (name ec) and click erase.
You obviously can't erase the disk that OS X has booted from, which is why you will need to start-up from the installer CD or summit.
Note: There are advanced options too, like Zeroing all data on the drive first, or writing over the drive numerous times to facilitate security.